The present invention generally relates to mixers. More particularly, the present invention relates to passive reflection mixers including single, double, and triple balanced mixers that employ one or more field effect transistors (FETs).
Mixers typically play an important role in radio frequency (RF) and microwave systems that employ frequency conversion. They are usually used to up-convert signal frequencies for transmission and down-convert received signals to lower intermediate frequencies (IF). Further in that regard, a mixer is generally considered a fundamental building block of a receiver.
Mixers may be broadly classified as either active type or passive type. The active mixer is frequently based upon the Gilbert-cell architecture, whereas the passive mixer is usually based upon a FET or diode ring. The active mixer approach typically yields low conversion loss or even gain, provides a good degree of balance, but often at the expense of modest linearity and the noise figure. On the other hand, the passive mixer is usually able to provide good linearity, but with typically 6 dB conversion loss and a need for high local oscillator (LO) drive levels.
The performance priorities of modern communication systems generally include stringent requirements for wide dynamic range, suppression of intermediation products, frequency stability and the effects of cross-modulation. These requirements must typically be carefully considered during the design of mixers.
In addition, due to crowding of the usable frequency spectrum and an increasing number of wireless applications, radio frequency receivers typically receive a growing number of unwanted signals at undesired carrier frequencies. The unwanted signals arrive at a receiver along with the information-bearing signal (i.e., the wanted signal) at the desired carrier frequency. When the unwanted signals are close in frequency to the carrier frequency of the information-bearing signal, a front-end receiver's filter typically cannot remove the unwanted interference signals. The unwanted signals then mix with each other and with the information bearing signals to generate intermodulation distortion (IMD). As such, minimization of the intermodulation (IM) products is typically an important goal.
Frequency conversion in mixers is generally regarded as being achieved through two different mechanisms. One mechanism is generally considered to have a non-linear characteristic, i.e., a square law characteristic, typically a diode V-I transfer characteristic or transistor transconductance characteristic. The other mechanism is generally considered to have a linear characteristic, i.e., time varying conductance, such as a switch pumped by a local oscillator (LO).
FETs generally exhibit a linear characteristic. In particular, a FET is generally considered linear based on the fact that a FET follows a square law characteristic (i.e., I∝V2) and therefore the first derivative, its transconductance, is expected to be constant. This is generally valid over a relatively wide amplitude range. FET mixers also typically require relatively low local oscillator power levels and provide isolation of the local oscillator signal relative to the radio and intermediate frequency signals. The overall performance characteristics of the FET double-balanced mixer have made it popular among available mixer types. Among its attributes are improved inter-port isolation and rejection of local-oscillator carrier amplitude modulation.
A FET mixer's intercept point is, however, subject to load-impedance variations, with a purely resistive termination typically providing the best case. Terminating the mixer with a filter is another possible approach to reducing load-impedance variations, but such a filter looks purely resistive only within its 3-dB pass-band; in the transition band and beyond, the filter impedance rises relatively rapidly and the mixer intercept point thereby degrades. Another approach is to configure the FET output as a high-pass filter, using capacitive coupling from the output tuned circuit to a 50-ohm band-pass filter. Outside its pass-band, this impedance inverter acts more like a short circuit and maintains IMD products at a reasonable level. The alternative to this is the popular diplexer, which requires more components and has more insertion loss.
An active FET mixer achieves gain at the expense of the intercept point; the difference can be as much as 20 dB. On the other hand, one can use any FET as a passive device by switching the source-drain channel ON and OFF. This impedance modulation is somewhat similar to a diode mixer, but the gate electrode is isolated from both source and drain. It nonetheless falls in the category of additive mixers because there is sufficient interaction between gate and source, although the impedance at the gate changes significantly less than in an additive diode mixer. Implementation of these types of mixers is usually challenging. Furthermore, building a high-performance passive FET mixer requires a pair or quad of mixer cells that should be sufficiently matched to suppress even-order IMD products.
Dual-Gate MOSFET/GaAsFET devices significantly suppress second order IMD products. In that regard, GaAs MESFET transistors have become popular for applications that require wideband performance, high dynamic range, and low distortion with small levels of LO power. In general, the 1-dB compression point, which is a commonly used figure-of-merit measurement for linearity in a mixer, is typically several dB above the LO drive level (3 to 6 dB) for a GaAs MESFET mixer. In comparison, a Schottky diode mixer is typically several dB below the LO drive level (3 to 6 dB). Therefore, improvements in mixer performance are generally achieved when MESFET transistors replace Schottky diodes in high linearity, low distortion mixers.
For example, in an article by Stephen A. Mass, entitled “A GaAs MESFET Mixer With Very Low Intermodulation,” IEEE Transactions on Microwave Theory Techniques, MTT-S Digest, 1987, pp. 895-898, one type of resistive mixer for high frequencies is described. The mixer is a balanced resistive type that uses the unbiased channel of a GaAs MESFET as the mixing element and operates at X-band frequencies.
Another article, by Ed Oxner, entitled “High Dynamic Range Mixing With the Si8901,” March 1988 Issue Of Ham Radio (and similarly in Siliconix Application Note AN85-2), shows a double balanced commutation-type MOSFET mixer constructed in a bridge configuration with four MOSFETS. This mixer is limited to operation in the HF and low VHF region. This mixer suffers a disadvantage in requiring a large gate voltage in excess of +15 volts to achieve 77 dB of rejection of inter modulation distortion (IMD) products referred to the input signal. Furthermore, a negative supply is also necessary at the MOSFET substrates, adding to the need of dual polarity power supplies.
Reference may also be made to an article by S. Weiner et al., entitled “2 to 8 GHZ Double Balanced MESFET Mixer with +30 dBm Input Third Order Intercept,” IEEE MTT-S Digest, 1988, pp. 1097-1099. This article describes a double balanced MESFET mixers operating in unbiased mode with +23 dBm LO drive level. Weiner's mixer typically has a third order input intercept (IIP3) of +30 dBm. Further reference may be made to Pavio, A. M. et al., “Double Balanced Mixers Using Active and Passive Techniques,” IEEE Transactions on Microwave Theory Techniques, MTT-36, 1988, pp. 1948-1956.
In addition, prior art balanced passive reflection FET mixers designed for use in high speed technologies at relatively high frequencies typically include directly connected gates as shown in FIGS. 1A and 1B. As shown in FIG. 1A, a prior art quasi-double balanced passive reflection mixer includes switching circuit 15 in series with a local matching circuit 19, local oscillator input circuit 23, RF-to-IF coupling circuit 25, diplexer circuit 29, and bias circuit 35. The mixer circuit also includes RF ports 41 and 47. Switching circuit 15 acts as the LO/RF or LO/IF signal mixer by creating a time varying impedance that varies in response to the applied LO signal.
The switching circuit 15 includes first (Q1) and second (Q2) FET transistors having their gates and sources electrically connected together as shown. The switching circuit 15 is a three port device having an input port for receiving a LO signal through the matching circuit 19 from the local oscillator circuit 23. The LO signal functions to operate the switch and the RF and IF ports 41, 47 through the RF-to-IF (combiner/transformer) circuit 25 and diplexer circuit 29. More particularly, the switching circuit 15 functions as the LO/RF or LO/IF signal mixer by creating a time varying impedance that varies in response to the LO signal. The diplexer circuit functions to filter and separate the IF signals from the RF signals during either up-frequency conversion or down-frequency conversion. In particular, in up-frequency conversion an IF signal is applied to port 47 and an RF signal is extracted at port 41. In down-frequency conversion, an RF signal is applied to port 41 and an IF signal is extracted from port 47. The RF-to-IF circuit 25 functions to prevent LO signals from appearing at the RF and the IF ports. The FET switch 15 is responsible for mixing the local oscillator signal from block 23 with the radio frequency signal coupled to the FET switch through the diplexer circuit 29 and RF-to-IF coupling circuit 25.
FIG. 1B is a simplified circuit diagram of a mixer of the type shown in FIG. 1A in common mode for the LO signal. From FIG. 1B the drain voltage may be shown to have a transition between VD(off) [VD(off)=VD, IN off state condition)] and VD(on) [VD-IDRD, IN On state condition] where VD is supply voltage. The resistor RD limits the DC current passing through the transistor and thereby serves as a load for the LO signal. In this configuration, a higher drain voltage is needed to achieve required linearity.
Mixers of the type shown in FIGS. 1A and 1B generally suffer from reduced efficiency due to coupling losses and instability due to the high frequency devices. Furthermore, because the circuit topology amplifies the signal from the local oscillator 23, special termination is often needed to achieve good performance. The circuit also typically requires a higher operating DC voltage and consumes a higher current. In addition, isolation of the local oscillator 23 is often poor due to the asymmetry of the circuitry (e.g., sources are grounded and the transformer that typically forms RF-to-IF circuit 25 is connected to load at its center tap).
Of utility then are mixers that improve the high frequency drive level transfer to the mixing cell as well as the low frequency stability, better isolate the local oscillator, and that provide a reduced level of non-linearity and intermodulation distortion.